One of the important types of rotary drill bits in the petroleum business is the roller cone bit. As the drill bit rotates, the applied weight-on-bit (“WOB”) forces the downward pointing teeth of the rotating cones into the formation being drilled. Thus the points of the teeth apply a compressive stress which exceeds the yield stress of the formation, and this induces fracturing. The resulting fragments are flushed away from the cutting face by the drilling fluid or mud.
Roller cone-type bits typically include bearings, such as ball bearings, roller bearings, or more simply journal bearings. A seal, such as an elastomeric seal, is typically used between the bearings and the outside environment to keep lubricant around the bearings and to keep contamination out. In a rotary seal, where one surface rotates around another, some special considerations are important in the design of both the seal itself and the seal gland into which it is seated. For instance, the elastomeric seal should be under compressive (as opposed to tensile) stress, and while there should be enough pressure between the seal and the rotating surface to prevent leakage, the pressure should be minimized to reduce friction and wear.
The constraints on the seals used in downhole applications are different from those of other low-speed sealing applications in several respects. First, everything in a bit, which operates deep in the earth, must be extremely robust to withstand the pressure and eccentric motion to which the bits are subjected. Additionally, the seals are themselves exposed to abrasive materials from two sources: not only does the drilling fluid near the cutting face include a heavy load of abrasive material (which is moving very turbulently at very high velocities), but the bearings themselves, as they wear, will tend to produce metal particles, and these metal particles themselves may be abrasive to a soft seal. Thus, both sides of the seal should ideally be protected from these abrasive effects. Additionally, the bit is operating in a remote environment from which it may take hours to retrieve for replacement, so it is highly desirable to have the bit operate for as long as possible.
One problem with conventional drill bit seals is that, as the bit is operated, the seal will inevitably wear, so that less compressive force is applied against the moving surface, running the risk that a leak will develop across the seal. Therefore, it is a desire to provide a seal assembly of a earth boring bit that protects the seal and improves bit performance.